Technical Field
The present invention generally relates to injection molded soldering and, in particular, to the use of mask cavities to reduce the pressure needed to inject solder.
Description of the Related Art
Injection molded soldering injects molten solder into vacuumed cavities on a patterned resist mask. Referring now to FIG. 1, a cross-sectional view of a conventional injection molded soldering process is shown. A contact pad 108 represents the under-bump metal (UBM) on a substrate 102. A resist mask 104 is formed around the contact pad 108, with a gap over the contact pad 108. The injection head 106 passes over the resist mask 104, injecting solder 110 into the cavity.
However, residual gas may remain in the cavity and have a negative effect on how the solder 110 fills the cavity. For example, bubbles may remain in the solder that weaken the solder joint and/or impede conductivity. When the molten solder makes contact with the UBM 108, residual gas can be removed by wetting of the solder on the UBM 108. If no such contact is made, then the residual cannot be removed by such wetting. It is difficult to remove this gas entirely, because there is often leakage between the solder injection head 106 and the resist mask 104.
One way to improve solder contact is to increase the injection pressure. However, higher injection pressure necessitates a correspondingly high injection head pressure, which can negatively affect the solder formation by deforming the mask 104. This puts an upper limit on how much pressure can realistically be applied by the injection head. For finer pitch bumping, even higher injection pressures are needed.